Over heating 1966 mustang running hot. 289 4/b. Have changed the thermostat, cap, and installed a overflow tank. What next?

I agree with the man that said to change the radiator hoses. I've noticed that they no longer put a spring in the lower hose to keep it from collapsing. I've saved the spring from an old hose and use it. Also, for me the recovery tank was not a good idea. At the time that I installed one I coincidentally started popping the radiator top tank open. After many times of this, I have just taken the tank off. Apparently, it needs that air cushion. ! use a 13 psi cap.

Have you checked the heater core try bypassing it .

I would imagine that the radiator hoses have been replace. If not, you might try it. They can look fine on the outside, but under load, the inside can collapse and cause a restriction to the circulation of the water. Been there, done that!
Cheers!
Ron

I'd look into cleaning out the radiator. Also, check for bad pullies and bearings in the belts. Those could add bad stress as well.

Also, check into flushing the coolant system. It may be full of corrosion and the like and having a rough time cooling. A flush is easy to do, and you can do it yourself. I recommend using an additive for coolant system flushing.

Try those ideas and see if that helps out. Another good thing to do is practice yearly blowouts on the radiator with high pressured air. From the inside out. This will help keep the radiator clear and allow the maximum airflow/cooling.

Hope these idea's help out man. Those old mustangs are back from the era when the mustang was a true demon of the road! :)

Check timing.... radiator flow..... water pump..... head gasket ? check for pressure in the radiator-water out the tailpipe good luck